In SS-7 or Clear Channel signaling networks, idle codes are issued by a central office switch over inter-machine trunks to indicate no connection or no voice frequency energy present. Since no signaling bits are available in SS-7 or Clear Channel signaling networks, idle codes are used by an echo canceler to detect off-hook and end of call conditions. However, when the inter-machine trunk is not in an idle state, i.e. such as when an off-hook condition exists, but the input line of the trunk has no voice frequency energy present, idle codes are interspersed with quiet/low noise and other non-idle codes. Typically, idle codes differ from non-idle codes by only the least significant bit. Conventional echo cancelers typically detect for idle codes only and, thus, false detection may occur on non-idle silent lines to force the echo canceler to re-enable.
An example of false detection and undesirable re-enable of the echo canceler may occur during facsimile data transmission when multiple pages are sent. During inter-page gaps, idle codes are interspersed with the non-idle codes. The echo canceler will detect the idle codes and re-enable, resulting in interference with the facsimile data transmission. Similar circumstances may occur during half duplex low speed modem data transfers. Therefore, it is desirable to have an echo canceler that does not falsely detect idle codes during non-idle operation.